In an IEEE 802.11 wireless local access network (WLAN), the communication channel is shared by multiple stations, which are coordinated by a distributed channel access (DCA) function, e.g., a distributed coordination function (DCF), based on the asynchronous distributed carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. A station with a data frame for transmission first performs a clear channel assessment (CCA) by sensing the wireless channel for a fixed duration, e.g., the DCF inter-frame space (DIFS). If the wireless channel is busy, the station waits until the channel becomes idle for a DIFS, and then waits for a further random backoff period. The backoff timer decreases by one for every idle slot and freezes when the channel is sensed busy. When the backoff timer reaches zero, the station starts data transmission.
If the data is received successfully, the receiver sends an acknowledgement (ACK) frame to the transmitter. If the transmitter does not receive an ACK during a certain interval, the transmitter retransmits the data frame after a random backoff. Note that each time a frame is not acknowledged, the transmitter doubles its backoff window until the maximum window is reached, which is referred to as exponential backoff.
In current WiFi, carrier sensing and exponential backoff are used for collision avoidance. Carrier sensing can efficiently prevent stations (STAs) within each other's sensing coverage from transmitting simultaneously. However, it does not work well when transmission STAs are hidden from each other. In high density WiFi with a large number of stations, severe collisions from multiple access can significantly degrade the network performance. By doubling the contention window size each time a collision occurs, the collision probability can be efficiently reduced when a large number of STAs are contending for channel access. However, a doubled backoff window also leads to a longer access delay of STAs, which degrades the quality of service (QoS) performance of STAs to some extent.
In addition, each time a collision occurs, the involved colliding STAs will re-attempt to transmit the same data again after a random backoff. Thus, collisions lead to inefficient channel utilization by wasting the channel air time due to retransmissions from all parties as well as a waste of battery power due to multiple retransmissions. The channel utilization efficiency of WiFi system is very low, e.g., less than 40%, due to the collisions and the involved collision avoidance overheads reflected by idle channel, including interframe spaces (e.g., DIFS and short interframe space (SIFS)) and backoff time.